Device for providing a circuit with resistive and capacitive characteristics where the resistive characteristic is controllable by electromagnetic radiation

ABSTRACT

A dielectric body composed of mercuric iodide is placed between and in contact with two capacitor plates. Electrical conductivity of mercuric iodide increases with the intensity of light on the dielectric body. The dielectric body then forms a resistive path between the two capacitor plates. The two plates and the dielectric body therefore form the circuit equivalent of a capacitor in parallel with a resistor, where the resistance of the resistor is that of the dielectric body. The resistance of the body may be controlled by controlling the intensity of incident light. Charges on the capacitor plates may be discharged by illuminating the body with light of adequate intensity.

The Government has rights in this invention pursuant to contract No.DE-AC08-83N10282 EG&G.

BACKGROUND OF THE INVENTION

This invention relates to a device for providing a circuit withresistive and capacitive characteristics wherein the resistivecharacteristic is controllable by electromagnetic radiation. Theinvention is particularly advantageous for providing a lightdischargeable capacitor.

In many electrical instruments it is desirable to provide capacitorswhich can be discharged remotely without making electrical contact.Electrometers are used to measure current from current sources.Typically, an electrometer includes a capacitor which is charged by acurrent from the current source. After the current is measured, thecapacitor in the electrometer must be discharged so that theelectrometer can be used for measuring other current sources. In orderto discharge the capacitor, the conventional method is by mechanicalswitching. Mechanical switching is disadvantageous, however, sinceelectrical contacts in the mechanical switches may breakdown afterrepeated usage and that mechanical switches may be costly. It istherefore desirable to provide a capacitor which can be dischargedwithout electrical contacts. It may also be desirable to providecapacitors which can be discharged remotely since capacitors may beplaced at locations that are difficult to reach.

In certain applications, it may be desirable to provide a deviceequivalent to a capacitor placed in parallel to a resistor where theresistance of the resistor can be optically controlled.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a circuit with resistiveand capacitive characteristics where the resistive characteristic iscontrollable by electromagnetic radiation.

A further object of the invention is to provide a capacitor which can bedischarged without electrical contacts.

The above objects of the invention are solved herein as follows. Thedevice of the invention comprises two electrically conductive elementsspaced apart from each other suitable for holding electrical chargesthereon. The device further comprises a dielectric body composed ofmercuric iodide between and in contact with the two elements. The bodyand the two elements form a capacitor. The body also forms a charge anddischarge resistive path between the two elements, where the path is inparallel to the capacitor. The path has an electrical conductance whichincreases with the intensity of electromagnetic radiation supplied tothe body. The dielectric constant of the body, however, remainssubstantially unaffected by the intensity of electromagnetic radiationsupplied to the body. The device also comprises means for supplyingelectromagnetic radiation to the body to control its electricalconductance.

By controlling the intensity of electromagnetic radiation on the body, acircuit is provided which has a substantially constant capacitance but acontrollable resistance. To provide a capacitor dischargeable withoutelectrical contacts, the conductance of the body and the intensity ofthe radiation supplied are such that the charges on the two elements aredischarged through the body.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of an electrometer which includes acapacitive and resistive device and a light source with associatedcircuitry to illustrate the preferred embodiment of the invention.

FIG. 2 is a perspective view of one implementation of the device of FIG.1 to illustrate the preferred embodiment of the invention.

FIG. 3 is a graphical illustration of the intensities of radiationsupplied by the light source of FIG. 1.

FIG. 4 is a schematic circuit diagram of a capacitive and resistivedevice and a light source illustrating another aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the recognition that when a dielectricbody composed of mercuric iodide is placed in between and in contactwith two capacitor plates, the dielectric body forms a charge anddischarge resistive path between the two plates, resulting in a devicewhich is equivalent to a capacitor is parallel with a resistor. When thedielectric body is then supplied with electromagnetic radiation such aslight, the dielectric constant of the body does not change but itselectrical conductance increases with the intensity of theelectromagnetic radiation. In this manner the device comprising the twocapacitor plates and the body forms a capacitor in parallel with aresistor whose resistance decreases with the intensity ofelectromagnetic radiation supplied. The resistance of the resistor canthen be controlled by varying the intensity of the radiation supplied.

FIG. 1 is a schematic view of an electrometer which includes acapacitive and resistive device and a light source with associatedcircuitry to illustrate the preferred embodiment of the invention. Inreference to FIG. 1, electrometer 4 includes an operational amplifier 6,device 10, light source 20 and power supply 30. Device 10 has bothcapacitive and resistive characteristics. As shown in FIG. 1, device 10is placed in a negative feedback path of amplifier 6, whosenon-inverting input is connected to ground. A signal current is also fedto the inverting input of the amplifier. The signal current chargesdevice 10 and appears at the output 8 of the electrometer as a voltage.

After the signal current is measured as a voltage appearing at terminal8, it is desirable to discharge device 10 so that electrometer 4 may beused to measure other signal currents. In conventional electrometers,device 10 usually comprises a capacitor and a mechanical switch used todischarge the capacitor. As explained above, the use of mechanicalswitches is disadvantageous. The construction of device 10 is describedin more detail below.

In reference to FIG. 1 device 10 comprises two electrically conductiveelements 12 and 14 spaced apart from each other suitable for holdingelectrical charges thereon. In the preferred embodiment elements 12 and14 are capacitor plates. A dielectric body 16 composed of mercuriciodide is placed in between and in contact with elements 12 and 14, thebody and the two elements forming a capacitor. Body 16 also forms acharge and discharge resistive path between the two elements. Device 10is therefore the circuit equivalent of a capacitor (with elements 12,14, 16) in parallel with a resistor whose resistance is that of body 16.While in FIG. 1 body 16 is shown to fill up the space between elements12 and 14, it will be understood that body 16 may take on other shapesas well, with the remaining space between the two elements taken up byother material. While such altered configuration changes the capacitanceof device 10, it does not alter the characteristic of device 10 in beingequivalent to a capacitor in parallel with a resistor.

Mercuric iodide has the property that its electrical conductanceincreases linearly with the intensity of light shining thereon, wherethe light should have wavelengths in the range of about 300-600nanometers. Mercuric iodide is a photoconductor with an external quantumefficiency of roughly 50% for photons with energy greater than thematerials ban-gap energy of about 2.1 eV. The time required for theconductance or the resistance of body 16 to change with the variation ofthe incident light intensity or level varies with the square of thethickness of body 16, inversely with the voltage applied to elements 12and 14, and inversely with the light level. In any given configurationof device 10, the response time is determined experimentally. Typicalresponse times might be in the range of a few microseconds.

In principle the conductance and resistance of body 16 will respond toessentially any light level and irrespective of whether the light is ata constant or varying level. To obtain useful results, however, it ispreferable for the light supplied by the light source to be at moderatelight levels, for example, at a few hundred microwatts per squarecentimeter. Thus, as shown in FIG. 1 light source 20 supplies light ofthe appropriate wavelength and intensity to body 16.

Mercuric iodide, as a dielectric material, has the advantage that itsdielectr1c constant remains substantially unchanged with the intensityof the incident light, in contrast to its conductivity. Capacitance ofdevice 10 may be increased by either reducing the thickness of body 16(which also reduces response time) or increasing the surface area ofdevice 10, or both.

As illustrated in FIG. 1, one particularly advantageous use of device 10is as a capacitor which can be discharged without making any electricalcontacts. Thus, elements 12 and 14 may be charged by means of a signalcurrent. The charges on elements 12 and 14 can be discharged bysupplying light to body 16 using light source 20. When light of asufficient intensity (such as a few hundred microwatts per squarecentimeter) is supplied to body 16, the charges on elements 12 and 14are discharged. Thus, device 10 is discharged without making or breakingany electrical contacts which may cause electrical noise. Such featuresare particularly desirable for applications where avoiding electricalnoise is important. To prevent ambient light from unintentionallydischarging device 10, it is preferable to enclose the device and lightsource 20 in an opaque container 21 shown in dotted lines in FIG. 1.Where device 10 is used as a light dischargeable capacitor, device 10 iseither kept in darkness to prevent discharge or illuminated with lightof sufficient intensity to discharge it, as illustrated above inreference to an electrometer.

FIG. 3 is a perspective view of an implementation of device 10 of FIG. 1to illustrate the preferred embodiment of the invention. As shown inFIG. 3, the implementation 10' of the device comprises two conductiveplates 12', 14' separated but in contact with a dielectric body 16' madeof mercuric iodide. Body 16' is elongated to maximize its surface areafor receiving light. In one implementation, body 16' is about 1 cm. inlength between plates 12', 14', where plates 12', 14' are each about 0.1sq. cm. in area. When a current of a few milliamperes is supplied to aLED light source supplying light to device 10', device 10' is dischargedin about 2 microseconds. The light emitted by the LED is about 600nanometers in wave length.

While mercuric iodide in the form of single crystals have been found tobe satisfactory for the purposes described herein, it is believed thatpolycrystaline mercuric iodide may also be used. Mercuric iodide usedshould be in tetragonal form (red phase) which is the stable, usableform at room temperature. Single crystals are vapor grown (by condensinga vapor into crystals in the red phase) and then sliced into wafers.Single crystal mercuric iodide can be obtained from EGG EMG, Post OfficeBox 1912, Las Vegas, Nev. 89125.

For the purpose of providing a capacitor dischargeable without makingelectrical contacts, mercuric iodide is a particularly desirablematerial to be used as a dielectric. The resistivity of mercuric iodidein the dark is about 10¹³ to 10¹⁴ ohm-cm, which is higher than those ofmany other light sensitive materials. For example, the dark resistivityof mercuric iodide is believed to be several orders of magnitude higherthan that of selenium. The response time of device 10 using mercuriciodide for body 16 is also faster than when many other types ofmaterials are used for body 16.

Light source 20 may be any appropriate light source such as a LEDsupplied with current from a power supply 30. As indicated earlier, thelight from light source 20 may be of a constant or varying (such aspulsed) level as illustrated in FIG. 2. To cause a LED to emit light ofa constant or varying level, power supply 30 supplies a constant orvarying current to the LED.

When device 10 is used as a circuit with a capacitor with constantcapacitance and a resistor with variable resistance placed in parallelas shown in FIG. 4, the intensity of light from light source 20 is usedto control the value of the resistance of the resistor (equal to that ofbody 16) in such equivalent circuit. The capacitance of the capacitor insuch equivalent circuit is equal to that of device 10. Device 10 ischarged by power supply 22 and the current through it measured by meter32. As elaborated above, the capacitance of device 10 does not changewith changes in the intensity of light from source 20. The precisecharacteristics of light intensities required to give the desiredresistance values are determined experimentally with a givenconfiguration of device 10. Light intensities varying in a desirablemanner (such as one shown in FIG. 2) may be used to obtain a desiredcapacitive and resistive circuit characteristic where the conductance ofthe circuit will vary in a manner similar to that of light intensity.

The invention is also advantageous for applications in resettable sampleand hold circuits, pico-ammeter integrators, current to frequencyconverters and pulsed optical feedback charge sensitive preamplifiers.

The above description of the invention is merely illustrative thereofand various changes in the materials and the details in the constructionand method may be within the scope of the appended claims.

What is claimed is:
 1. A system comprising:a circuit which includes adevice with resistive and capacitive characteristics wherein theresistive characteristic is controllable by exposure to electromagneticradiation, said device comprising: (a) two electrically conductiveelements spaced apart from each other suitable for holding electricalcharges thereon; and (b) a dielectric body composed of mercuric iodidebetween and in contact with the two elements, the body and the twoelements forming a capacitor, said body also forming a charge anddischarge resistive path between the two elements and in parallel to thecapacitor, said path having an electrical conductance which increaseswith the intensity of electromagnetic radiation supplied to the body andwherein the dielectric constant of the body remains substantiallyunaffected by the radiation intensity; and means for supplyingelectromagnetic radiation to the body to control its electricalconductance, thereby controlling the circuit without making or breakingany electrical contacts.
 2. The system of claim 1, wherein the body issuch that when electromagnetic radiation of wavelengths substantiallywithin the range 300 to 600 nanometers is supplied to the body, chargesstored in the two elements are discharged through the body, so that thedevice is dischargeable without making or breaking electrical contacts.3. The system of claim 2, wherein the body is such that said device isdischargeable by electromagnetic radiation of a few hundred microwattsper square centimeter in intensity.
 4. The system of claim 1, whereinsaid body is composed of single crystals of mercuric iodide.
 5. Thedevice system of claim 4, wherein said single crystals are vapor grown.6. The system of claim 1, wherein said body is composed ofpolycrystaline mercuric iodide.
 7. The system of claim 1, wherein saidmeans for supplying electromagnetic radiation is electrically insulatedfrom the device and supplies radiation to the body of such intensitythat charges stored on the two elements are discharged through the body,so that the device is dischargeable without electrical contacts.
 8. Thesystem of claim 7, wherein said means for supplying electromagneticradiation supplies light of a few hundred microwatts per aquarecentimeter in intensity.
 9. The system of claim 1, wherein said meansfor supplying electromagnetic radiation supplies light of wavelengthssubstantially in the range 300-600 nanometers.
 10. The system of claim1, wherein said means for supplying electromagnetic radiation suppliesphotons to the body, and wherein at least some of the the photons haveenergies greater than about 2.1 ev.
 11. A method for discharging acapacitor, said capacitor including two electrically conductive elementsspaced apart from each other and suitable for holding electrical chargesthereon, and dielectric body composed of mercuric iodide between and incontact with the two elements, wherein the electrical conductance of thebody increases with the intensity of electromagnetic radiation suppliedto it and wherein the dielectric constant of the body remainssubstantially unaffected by the radiation intensity, said methodcomprising supplying electromagnetic radiation to the body of suchintensity that charges stored on the two elements are discharged throughthe body, so that the capacitor is dischargeable remotely and withoutelectrical contacts.
 12. The method of claim 11, wherein the radiationsupplying step supplies radiation of a substantially constant intensity.13. The method of claim 11, wherein the radiation supplying stepsupplies radiation of a varying intensity.
 14. The method of claim 13,wherein the radition supplying step supplies radiation of a pulsedwaveform.
 15. The method of claim 11, wherein the radiation supplyingstep supplies radiation of a wavelength substantially in the range of300-600 nanometers.